顧名思義, numeric 是一個用於數值計算的小庫.
Generalized numeric operations
This header describes a set of algorithms to perform certain operations on sequences of numeric values. Due to their flexibility, they can also be adapted for other kinds of sequences.

比較常用的就一個累加函式 accumulate, 定義和實現如下

template <class InputIterator
 
, class T>
   T accumulate (InputIterator first, InputIterator last, T init)
{
  while (first!=last) {
    init = init + *first;  // or: init=binary_op(init,*first) for the binary_op version
    ++first;
  }
  return init;
}

場景, 計算平均值

#include <numeric>
...
std::vector<float> scores(3)
 
;
scores.push_back(1.2);
scores.push_back(2.8);
scores.push_back(3.5);

float mean = std::accumulate(scores.begin(), scores.end(), 0.0) / scores.size();
...

此外, 還有幾個函式, 也比較有意思

• inner_product, 計算兩個向量的內積

template <class InputIterator1, class InputIterator2, class T>
   T inner_product (InputIterator1 first1,
 
 InputIterator1 last1,
                    InputIterator2 first2, T init)
{
  while (first1!=last1) {
    init = init + (*first1)*(*first2);
               // or: init = binary_op1 (init, binary_op2(*first1,*first2));
    ++first1; ++first2;
  }
  return init;
}

---
  int init = 100;
  int series1[] = {10,20,30};
  int series2[] = {1,2,3};

  std::cout << "using default inner_product: ";
  std::cout << std::inner_product(series1,series1+3,series2,init);
  std::cout << '\n';
  => using default inner_product: 240

• adjacent_difference : 計算臨差

template <class InputIterator, class OutputIterator>
   OutputIterator adjacent_difference (InputIterator first, InputIterator last,
                                       OutputIterator result)
{
  if (first!=last) {
    typename iterator_traits<InputIterator>::value_type val,prev;
    *result = prev = *first;
    while (++first!=last) {
      val = *first;
      *++result = val - prev;  // or: *++result = binary_op(val,prev)
      prev = val;
    }
    ++result;
  }
  return result;
}

---
  int val[] = {1,2,3,5,9,11,12};
  int result[7];
  std::adjacent_difference (val, val+7, result);
  std::cout << "using default adjacent_difference: ";
  => using default adjacent_difference: 1 1 1 2 4 2 1

• partial_sum:計算區域性累加和

template <class InputIterator, class OutputIterator>
   OutputIterator partial_sum (InputIterator first, InputIterator last,
                               OutputIterator result)
{
  if (first!=last) {
    typename iterator_traits<InputIterator>::value_type val = *first;
    *result = val;
    while (++first!=last) {
      val = val + *first;   // or: val = binary_op(val,*first)
      *++result = val;
    }
    ++result;
  }
  return result;
}
---
  int val[] = {1,2,3,4,5};
  int result[5];

  std::partial_sum (val, val+5, result);
  std::cout << "using default partial_sum: ";
  => using default partial_sum: 1 3 6 10 15

• iota: Store increasing sequence 生成遞增數列, 很簡單的功能, 很奇怪的名字.

template <class ForwardIterator, class T>
  void iota (ForwardIterator first, ForwardIterator last, T val)
{
  while (first!=last) {
    *first = val;
    ++first;
    ++val;
  }
}
---
  int numbers[10];
  std::iota (numbers,numbers+10,100);
  std::cout << "numbers:";
  => numbers: 100 101 102 103 104 105 106 107 108 109

Ref